Polyamides are today used in a multiplicity of areas, such as for textiles, carpets or cords; in the production of injection moldings, such as in the automotive area; or in the production of plastic films or blown containers for packaging. In this connection, the multiplicity of positive properties is of particular significance, including the high durability and high thermal resistance combined with the good elasticity of polyamides.
An overview of various methods for the production of polyamides, including caprolactam-based polyamide 6 or copolyamides, is provided in Kunststoff-Handbuch [Plastics Manual], Volume 3 “Technische Thermoplaste” [Technical Thermoplastics], Subvolume 4 “Polyamide” [Polyamides], published by G. W. Becker and D. Braun, Hanser-Verlag Munich and Vienna, 1998, pages 22 to 75. An overview of spinning applications of polyamides can be found, for example, in “Synthetische Fasern” [Synthetic Fibres], Handbuch für Anlagenplanung [Manual of Plant Engineering], Franz Fourné, Hanser-Verlag Munich and Vienna, 1995, pages 36 to 56.
Generally, in established methods and devices for the production of polyamides, especially for the production of polyamide 6 or copolyamides, a caprolactam-based melt is subjected to polymerization. Accordingly, the produced polyamide melt is pelletized, for example by means of underwater pelletization or by means of strand pelletization. Such a pelletized polyamide material, however, usually still contains around 10% of low-molecular species or components, such as caprolactam and cyclic oligomers. In order to prevent a disruption by such low-molecular components during the further treatment of such a pelletized polyamide material, such low-molecular components are reduced by extraction to the level required for the particular application, the residual concentration thereof being thus adjusted. Conventionally, for this purpose, the pellets, which are dried after pelletization, are subjected to aqueous extraction, in the course of which the pellets conventionally take up considerable quantities of water. Consequently, such an extraction stage is followed by a further drying stage. The prior art makes use, for example, of inert-gas dryers, which are arranged downstream of a countercurrent extraction device.
The water at the outlet of the extraction stage typically has an extract concentration, i.e. a concentration of low-molecular components, of over 10% by weight. This extraction water is either returned directly to the polymerization process or is reprocessed after extraction. After extraction of the low-molecular components, the extracted low-molecular components are recovered and can then be returned to the polymerization process.
With regard to the above-described pelletization process, it must generally be ensured that there is compliance with a composition of the pelletization water that is suitable for a reliable pelletization process, with the extract concentration in the pelletization water normally being around 4% by weight. If this is not the case, the pelletization water cannot be optimally treated, because there is a foaming of the pelletization water.
The direct supply of pelletization water to the extraction stage could undesirably lower the extract concentration contained therein. The required degree of dryness of the polyamide pellets (commonly referred to as PA chips) at the inlet to the extraction stage is around 1% by weight.
Publication WO 02/094908 A1 describes a method and device for the extraction of polyamide wherein the pelletization of polyamide melt into pellets is followed by a separate extraction of the low-molecular (residual) components using a special extraction water under specific conditions, more particularly in a plurality of stages at increased temperature and pressure.
The object of the present embodiments are to overcome the disadvantages of the prior art and to provide a simple, cost-effective, and reliable method and device for the production of pellets of polyamide 6 or copolyamides, and to produce pellets of polyamide 6 or copolyamides that are capable of being further processed in a simple manner.
The present embodiments are detailed below with reference to the listed Figures.